Many internal combustion engines comprise an injection device having injectors that inject fuel into the combustion chambers. The injectors or injection valves are controlled by a suitable control device in order to optimally proportion the fuel amounts. The fuel amounts are generally proportioned in a time-controlled manner, i.e. the injector is opened for a precisely determined period of time and is then closed again. This period of time is referred to in this instance as the control period of the injector.
A characteristic map is normally present in a control device of the internal combustion engine and this determines a desired injection characteristic curve, i.e. a correlation between injected fuel amounts and the control period. It is important to be able to adjust the injected fuel amounts accurately, so the internal combustion engine can function at its optimal working point.
An actual injection characteristic curve may deviate from the desired injection characteristic curve owing to production-related variations or age-related changes of the injectors or internal combustion engine. This means that the correlation between the control period and the injected fuel amount in the actual state may deviate from the desired state. Since the changes are normally very slight changes to the injection amounts, the absolute value of which is largely independent of the control period or injection amount, a minimum amount is often adapted in order to adapt the injection characteristic curve.
Adaptation of a minimum amount of this type is disclosed in greater detail in document DE 102 57 686 A1. In this case an injection valve characteristic curve of a controlled injector of an internal combustion engine reproducing a single-reference injection behavior is adapted to age-related changes of an actual injection behavior in that the injection valve is controlled intermittently in accordance with a control period during an operating mode of the internal combustion engine that does not require fuel injection, whilst otherwise no fuel is injected so at least one working cycle of the internal combustion engine with control follows or precedes at least one working cycle without injection valve control and in each case a speed value or a value of a speed-dependent variable of the internal combustion engine is detected for the working cycle with control and for at least one of the working cycles without control and a differential between the detected values is formed and therefore the injection characteristic curve is corrected.
The method from the aforementioned document thus yields satisfactory results if the internal combustion engine is a diesel engine. A drawback of this method is therefore linked to the fact that, in order to take into account the value measured during the working cycle with control, it is merely assumed that an injection has taken place. Since with petrol engines or internal combustion engines with active ignition an injected minimum amount of fuel does not necessarily have to be ignited, the method disclosed in the referenced document is not suitable for internal combustion engines with active ignition.